Adjust cache times based on various factors

[aaronpk]

• Tweets can't be edited, so they can be cached for a long time. (It's possible that Twitter may change their image CDN URLs so can't be infinite)
• Instagram posts can be edited, but most of the time the older a photo is the less likely it is to be edited
• Other URLs should be more conservative about being cached, since people sometimes use this for testing whether their markup changes are working, or it's used when verifying webmentions for updates. Could probably use similar logic to cache older posts longer.

Possibly give clients a way to force XRay to fetch the post if the client knows it may have been updated such as from a webmention update.